DEVIL PHYSICS THE BADDEST CLASS ON CAMPUS PRE-IB PHYSICS
Dec 28, 2015
TSOKOS LESSON 10-10 TO 10-13
10-10: VISCOSITY10-11: FLOW IN TUBES: POISEUILLE’S EQUATION, BLOOD FLOW10-12: SURFACE TENSION AND CAPILLARITY10-13: PUMPS: THE HEART AND
BLOOD PRESSURE
Viscosity
A friction force between adjacent layers of fluid as the layers move past one another
In liquids, it is mainly due to the cohesive forces between molecules
In gases, it is caused by collisions between molecules.
Coefficient of viscosity, η (lowercase eta) (Pa-s)
Viscosity
Determined by measuring the force required to move a plate over a stationary one with a given amount of liquid between them
Viscosity Determined by
measuring the force required to move a plate over a stationary one with a given amount of liquid between them
v
l
A
Fl
vAF
Viscosity Units for η (eta)
are N·s/m2 or Pa·s CGS is dyne·s/cm2
which is called a poise (P)
100 centipoise (cP) = 1P
v
l
A
Fl
vAF
Coefficients of Viscosity Temperatures are specified because it
has a strong effect on viscosity Viscosity for most fluids decreases
rapidly with increase in temperature
Flow In Tubes: Poiseuille’s Equation
Without viscosity, fluids could flow freely without an applied force
Because of viscosity, a pressure difference between the ends of the tube are necessary to cause the fluid to flow
Flow In Tubes: Poiseuille’s Equation
Rate of flow of a fluid depends on: Viscosity Pressure difference Dimensions of the tube
Poiseuille’s Equation assumes Fluid is incompressible Laminar flow
Flow In Tubes: Poiseuille’s Equation
Q is the volume rate of flow in m3/s
r is the inside radius of the tube L is the length of the tube P1-P2 is the pressure difference
between the ends η is the coefficient of viscosity
L
PPrQ
821
4
Flow In Tubes: Poiseuille’s Equation
Q, the volume rate of flow is Directly proportional to the pressure
difference Inversely proportional to the viscosity
and length of the tube Directly proportional to the fourth
power of the radius
L
PPrQ
821
4
Flow In Tubes: Poiseuille’s Equation
Q, the volume rate of flow is Directly proportional to the fourth power of
the radius Do blood vessels have constant
diameter?
L
PPrQ
821
4
Flow In Tubes: Poiseuille’s Equation
Q, the volume rate of flow is Directly proportional to the fourth power of the radius
Do blood vessels have constant diameter? Blood vessel diameter decreases as they branch out The body controls blood vessel diameter by bands of
muscles surrounding the arteries Arteriosclerosis and cholesterol buildup decrease
diameter forcing a higher pressure gradient for same blood flow
L
PPrQ
821
4
Surface Tension
Because of the cohesion of liquid molecules, the surface of a liquid acts like a stretched membrane under tension Drip of a faucet Water beading on a surface Meniscus in a test tube
Surface Tension Surface tension,
γ, is defined as the force, F, per unit length, L that, across any line in a surface, tends to pull the surface closed
L
F
Surface Tension Because of surface
tension, insects can walk on water and objects more dense than water can ‘float’
L
F
Surface Tension Soaps and
detergents contain substances called surfactants that decrease the surface tension of water Allows the water to
penetrate crevices and between fibers
L
F
Capillarity
Water in a glass will tend to ‘stick’ to the glass and slope upward at the edges toward the glass
Mercury, on the other hand, tends to ‘stick’ more to itself and slopes downward toward the glass
Sloping upward or downward is a function of the liquid’s cohesion and adhesion properties and the surface it is in contact with
Capillarity
Cohesion refers to the attractive force between molecules of the same type (the liquid)
Adhesion refers to the attractive force between molecules of different types (the liquid and the glass)
Capillarity
In tubes of relatively small diameter, liquids will either rise or fall relative to surrounding fluid based on its adhesive and cohesive properties Fluids with strong adhesive properties
will rise Fluids with strong cohesive properties
will fall The amount of rise or fall depends on
the surface tension Capillarity Demonstration
Important News
Because of time constraints, battle fatigue and PTSD, there are no homework problems for 10-12 and 10-13
Concepts from 10-12 and 10-13 (i.e. Reading Activity) are testable
Read through section 10-13 on your own and pay special attention to the Reading Activity!